What source can he give you? It's clearly a personal annecdote that is shared between a lot of people that use vim. What kind of source are you thinking about?
Are you saying that studying how efficient various text editors are for some groups of people is not possible?
Here's what I believe to be an example of some study on how text editors compare:
https://mjambon.github.io/vim-vs-emacs/
Not saying that this is a good study - I haven't even read it. Merely providing it as an example that at some point a study has been made in this area.
> Assuming you've given time to learn it, editing with vim takes less mental effort and is faster.
What gave away that it was a personal anecdote? I interpreted the "you" part as if he was speaking generally about other people and not only himself
In my main language, we would have written something like below if it was a personal anecdote (rough direct translation):
> I was able to edit faster using Vim after taking the time to learn it.